welcome to Pakwheels
with current prices of cng & petrol, running costs/KM are almost same on my mehran, and i have recently disconnected my car's CNG kit and removed car's CNG tank to reduce weight, and now the car is performing very good, better than any cng car of same category.
I will not recommend you to install a cng kit with current costs of petrol/cng.
only will recommend you to buy a kit if you get a good bargain, and keep it until cng costs is low, or petrol price is high again.